1. Field
This invention relates to a semiconductor pressure sensor. More specifically, the present invention is directed to a semiconductor pressure sensor of the type that uses strain gauges formed on a silicon diaphragm.
2. Background
Conventionally, a semiconductor pressure sensor using strain gauges is known. The pressure sensor forms a pressure-sense diaphragm on a silicon substrate. And, sensor elements (piezo-resistive devices) comprised by diffusion resistor layers are provided on the pressure-sense diaphragm. The variation of a pressure is measured by the detection of the distortion in the diaphragm.
FIG. 7 is a perspective diagram showing a semiconductor pressure sensor using conventional strain gauges. A part of the pressure sensor is shown by the cross section view. As shown in FIG. 7, sensor chip 200 is made by silicon base 101, which has diaphragm 110, and sensor elements on diaphragm 110. Diaphragm 110 provides the whole center section of silicon base 101 with a thin film, excluding a circumference part. Wheatstone Bridge circuit 113 is comprised by strain gauges 105a-105d made from diffusion resistors, metal wiring 103, and terminals 104a-104d. 
FIG. 8 is a circuit diagram showing Wheatstone Bridge circuit 113 based on FIG. 7. As shown in the diagram, strain gauges 105a-105d made from diffusion resistors are respectively connected by metal wiring 103. Terminals 104a-104d are provided between each strain gauge. Terminal 104a is connected to a power supply (high potential side). Moreover, terminal 104c is connected to a ground (low potential side). Therefore, a variation of resistance in strain gauges 105a-105d is performed by the deformation of diaphragm 110 of FIG. 7. The voltage value between terminals 104b and 104d varies. The variation of a pressure is measured by the detection of change in voltage.
In the meantime, sensor chip 200 is fixed on pedestal 111, such as Pyrex(copyright)glass. And, sensor chip 200 is sealed in a package together with a silicon sealing liquid. Pedestal 111 provides through-hole 112 for extracting air. Sensor chip 200 is attached so that through-hole 112 may be covered. The silicon sealing liquid (not illustrated) is maintained on diaphragm 110. The sensor elements on diaphragm 110 (each member which comprises Wheatstone Bridge circuit 113) is isolated from an external field. Therefore, the variation of a pressure is transmitted to the sensor elements via the silicon sealing liquid.
[Problem to be solved]
The pressure sensor (as shown above) needs a fine pattern process on the silicon substrate for formation of the diaphragm and the diffusion resistors, and is made from the semiconductor manufacturing process, which must be considered sufficiently dustproof. However, even though the present clean room provides means for preventing dust, a trace metal-impurity enters into a wafer or is generated midway through a process. As a result, the metal-impurity may bring on a fluctuation in a sensor output.
In general, when various semiconductor devices, such as MOSFET or the like, are manufactured, a removal in the influence of a device, etc. is performed by capturing the metal-impurity during the manufacturing process of the wafer. This is called gettering. From the difference in the principle, it is classified into a EG (extrinsic gettering) method and a IG (intrinsic gettering) method. The EG method is the technique that roughens a wafer back-side using a sandblasting method, etc. to collect the impurity in the roughened-surface. The IG method is the technique that makes many micro defects inside the wafer by precipitates of oxygen to capture the impurity in the micro defects.
However, the semiconductor pressure sensor with the structure that provides the strain gauges on the diaphragm etches most silicon-substrate back-sides to form the diaphragm. For this reason, even though gettering is performed in the wafer using the conventional EG and IG methods, a getter reduces at the time of a formation of the diaphragm. Therefore, it becomes difficult to capture the impurity sufficiently. Moreover, a new process for making the getter is required. There is also a problem that an effect changes with varieties of the wafer (a bare substrate, SOI (Silicon On Insulator) substrate, epitaxial substrate, etc.).
[Means for solving the problem]
The invention is made in order to solve the above-mentioned problem, and an object of the present invention is to provide a semiconductor pressure sensor in which fluctuation in a sensor output is difficult to be produced.
A semiconductor pressure sensor, according to the present invention, comprises Silicon substrate (1) with diaphragm (10) that produces a distortion depending on a pressure, strain gauges (5a, 5b, 5c, 5d) that are provided on diaphragm (10) and are formed by diffusion resistors, and a PN-junction area that is provided adjacent in strain gauges (5a, 5b, 5c, 5d) and that the reverse bias is applied to.
The PN-junction area may comprise the boundary surface between silicon base (1) and diffusion layer (8) provided in silicon base (1).
Diffusion layer (8) may be locally provided near strain gauges (5a, 5b, 5c, 5d).
A plural pair of strain gauges (5a, 5b, 5c, 5d) may be provided.
Plural strain gauges (5a, 5b, 5c, 5d) may form Wheatstone Bridge circuits.
The PN-junction area may be provided only in strain gauge (5c) at the side of the large electrical potential difference with a substrate potential among terminal (4a) at the side of a high electric potential in the Wheatstone Bridge circuit and the terminal at the side of low potential (4c).
Diffusion layer (8) may be formed of the combination of the plural long and slender patterns that are acutely angled toward strain gauges (5a, 5b, 5c, 5d).